If you add a newer drive with a higher speed you'll have to set that up as you master drive as otherwise you won't see the speed increase. SSDs are expensive and whilst quicker and more reliable it is probably worth looking at a new rig rather than wasting money on something that will get you only a very small gain in terms of performance.
There are two types of aeroplane, Spitfires and everything else that wishes it was a Spitfire!
Thanks for that ozzy, I was figuring a new rig would be the ultimate answer. My thinking was: If a seperate hard drive didn't give much benefit on the old rig I could then use it on a new rig, so hopefully no money wasted.
I think one advantage to having a separate drive or a partitioned drive is FSX won't fragment as fast. It's something I didn't do this time. I kind of wish I had, so when I defrag it doesn't take 6 hours. But I think Ozzie is right, it won't really improve much other than fragmentation.